1. You should get 15 frames on 120 and 30 frames on 220. It seems like you should get 16 and 32, like with higher end 645s, but you do not.
2. That is the D of F preview lever. It momentarily stops down the lens to the selected aperture, as opposed to the manual/automatic lever on the lens, which will keep the aperture stopped down without having to hold anything.
3. Yes. It is the timer. To find out how long it runs, just run it. It's pretty standard; perhaps 7 seconds. You cock it with the big lever, and release it with the small lever.
4. It takes a plain-ol' cable release on the lower shutter release.
FYI, the sub-1000S models do not have 2 and 3 (nor do they have a '1000 shutter speed or the interlock for changing shutter speeds).